2. Related Art
The clinical use of intracranial pressure monitoring was first described by Janny in 1950 and Lundberg in 1960.
Intracranial pressures may be measured by different strategies. Solid or fibre-optic transducers may be introduced into the epidural or subdural spaces, or introduced into the brain parenchyma. Intracranial pressure also may be recorded directly by measuring pressure in the cerebrospinal fluid, requiring application of catheter to the cerebrospinal fluid space (most commonly in the cerebral ventricles or the lumbar spinal cavity). During infusion tests the pressure in the cerebrospinal fluid is recorded.
The present invention deals with strategies to analyze single pulse pressure waves, and make analysis of these waves available to the daily clinical practice. The fluctuations of intracranial pressure arise from cardiac and respiratory effects. The intracranial pressure cardiac waves or cerebrospinal fluid pulse waves result from the contractions of the left cardiac ventricle. The intracranial pressure wave or the cerebrospinal fluid pulse wave resemble the arterial blood pressure wave, that is characterized by a systolic rise followed by a diastolic decline and a dicrotic notch. In addition, changes in pressures associated with the respiratory cycle affect the intracranial pressure wave. The morphology of the intracranial pulse pressure wave depends on the arterial inflow, venous outflow, as well as the state of the intracranial contents. The single pulse pressure waves of intracranial pressure include three peaks that are consistently present, corresponding with the arterial pulse waves. For a single pulse pressure wave the maximum peak is termed P1 or top of the percussion wave. During the declining phase of the wave, there are two peaks namely the second peak (P2), often termed the tidal wave, and the third peak (P3), often termed the dicrotic wave. Between the tidal and dicrotic waves is the dicrotic notch that corresponds to arterial dicrotic notch. In the present application, the amplitude of the first peak (ΔP1) is defined as the pressure difference between the diastolic minimum pressure and the systolic maximum pressure, the latency of the first peak (ΔT1) is defined as the time interval when pressures increases from diastolic minimum to systolic maximum. The rise time (ΔP1/ΔT1)) is defined as the coefficient obtained by dividing the amplitude with the latency. The morphology of the single pulse pressure wave is intimately related to elastance and compliance. Elastance is the change in pressure as a function of a change in volume, and describes the effect of a change in volume on intracranial pressure. Compliance is the inverse of elastance and represents the change in volume as a function of a change in pressure. Therefore, compliance describes the effect of a change in pressure on craniospinal volume. Elastance is most useful clinically as elastance describes the effect of changes in intracranial volume on intracranial pressure. The relationship between intracranial pressure and volume was described in 1966 by Langfitt and showed an exponential curve, where the slope of any part of the curve resembles the rise time of a single wave (that is ΔP/ΔT or change in pressure/change in volume). The curve is termed the pressure-volume curve or the elastance curve. The horizontal part of the curve is the period of spatial compensation whereas the vertical portion is the period of spatial decompensation. When elastance increases also the amplitude of a single pulse pressure wave increases due to an increase in the pressure response to a bolus of blood from the heart. It has, however, not been possible to take the knowledge of single wave parameters into daily clinical practice.
In the intensive care unit, continuous intracranial pressure monitoring usually presents the pressures as mean pressure in numerical values, or as a curve that has to be visually analyzed. Though single waves may be displayed on the monitor, strategies to explore trends in changes of single wave characteristics are lacking. Furthermore, strategies to continuously examine compliance solely on the basis of the pressure curves have not been established.
There is a close relationship between blood pressure and intracranial pressure as the intracranial pressure waves are built up from the blood pressure waves. Simultaneous assessment of intracranial pressure and blood pressure provides several advantages, for instance by calculation of the cerebral perfusion pressure (that is mean arterial pressure minus intracranial pressure). The assessment of cerebral perfusion pressure represents a critical parameter in the monitoring of critically ill patients. Assessment of blood pressure per se also has a major place in daily clinical practice, including both assessments of diastolic and systolic pressures.